Remotely controlled electrohydraulic system with fail-safe features

ABSTRACT

The system includes a hydraulic user device and a source of hydraulic fluid. A hydraulic arrangement connects the source of fluid to the user device and provides a path for the flow of fluid from the source to the user device, and includes a control member positioned in such path and mounted for movement between a plurality of positions in different ones of which the control member differently affects the flow of fluid from the source to the user device, including a neutral position in which the control member blocks the flow of fluid from the source to the user device. An electrohydraulic moving arrangement is operative when electrically energized for moving the control member by applying hydraulic force thereto. A restoring arrangement is operative for applying to the control member a restoring force tending to move the control member to said neutral position when the electrohydraulic moving arrangement is electrically unenergized. An electrical control arrangement controls the electrohydraulic moving arrangement and includes a source of electrical power having two terminals, connectors connecting the source of electrical power to the electrohydraulic moving arrangement for furnishing electrical energy to the latter, a remote control arrangement and an electrical control cable having a first cable end connected to the electrohydraulic moving unit and a second cable end connected to the remote control arrangement, the control cable comprising a first conductor having at the first cable end a first end connected to one terminal of the source of electrical power and having a second end at the second cable end, a second conductor extending along the length of the cable and having a first end at the first cable end and a second end at the second cable end. A fail-safe unit prevents electrical energization of the electrohydraulic moving arrangement when a short-circuit develops between the first and second conductors, to prevent the application by the electrohydraulic moving arrangement of hydraulic force to the control member and to thereby cause the control member to be returned to the neutral position by the restoring means.

United States Patent 1191 Arnold et a1;

[451 Aug. 27, 1974 [73] Assignee: Robert Bosch GmbH', Stuttgart,

Germany 22 Filed: Aug. 9, 1973 211 Appl. No.: 387,201

[30] Foreign Application Priority Data Aug. 18, 1972 Germany 2240607 52 U.S.C1. ..91/459,317/4s 51 1m. 01. FlSb 13/044 [58] Field of Search 317/123', 148.5 R, 148.5 B, 3l7/DlG. 7, 18 A, 18 D, 45, 596.16; 91/459,

[56] References Cited UNITED STATES PATENTS 3,390,387 6/1968 l-lugenholtz.... 317/l48.5 R 3,407,340 10/1968 Hufnagel 3l7/l48.5 R 3,602,772 8/1971 Hundhausen et a1. 317/45 3,613,509 10/1971 Flaschar et al. 91/459 Primary ExaminerL. T. Hix

Attorney, Agent, or Firm-Michael S. Striker [57] ABSTRACT The system includes a hydraulic user device and a source of hydraulic fluid. A hydraulic arrangement tioned in such path and mounted for movement between a plurality of positions in different ones of which the control member differently affects the flow of fluid from the source to the user device, including a neutral position in which the control member blocks the flow of fluid from the source to the user device. An electrohydraulic moving arrangement is operative when electrically energized for moving the controlmember by applying hydraulic force thereto. A restoring arrangement is bperaii'varar'a fiiyfii'g'ta the control member a restoring force tending to move the control member to said neutral position when the electrohydraulic moving arrangement is electrically unenergized. An electrical control arrangement controls the electrohydraulic moving arrangement and includes a source of electrical power having two terminals, connectors connecting the source of electrical power to the electrohydraulic moving arrangement for furnishing electrical energy to the latter, a remote control arrangement and an electrical control cable having a first cable end connected to the electrohydraulic moving unit and a second cable end connected to the remote control arrangement, the control cable comprising a first conductor having at the first cable end a first end connected to one terminal of the source of electrical power and having a second end at the second cable end, a second. conductor extending along the length of the cable and having a first end at the first cable end and a second end at the second cable end. A fail-safe unit prevents electrical energization of the electrohydraulic moving arrangement when a short-circuit develops between the first and second conductors, to prevent the application by the electrohydraulic moving arrangement of hydraulic force to the control member and to thereby cause the control member to be returned to the neutral position by the restoring means.

9 Claims, 2 Drawing Figures MEI 20F 2 PAIENTED M27 1974 Fig.2

I I L J107 1 REMOTELY CONTROLLED ELECTROI-IYDRAULIC SYSTEM WITH F AIL-SAFE FEATURES BACKGROUND OF THE INVENTION The present invention relates to an electrohydraulic control system having a-distributing piston moved by a differential-piston pilot arrangement provided with a biasing arrangement for biasing the differential-piston of the pilot arrangement to a predeterminedneutral position, with the flow of fluid into and out of the pilot arrangement being controlled by at least two electromagnetically actuated two-port two-position valves controlled remotely from a remote control arrangement provided .with at least two controlswitches and connected to the electrohydraulic pilot arrangement by means of a control cable.

It is of course known to provide electrohydraulicarrangements which must be controlled by the operator at the actual location of the arrangement. Likewise, electrohydraulic arrangements are known which can be controlled'at distance, by means of a control cable. However, when such arrangements are remotely controlled in this way, there always exists the dangerous possibility of damage to the control cable, which may result in uncontrolled operation of the electrohydraulic system. Such uncontrolled operation may cause injury to nearby personnel or cause damage to mechanical components, goods being processed, and the like.

SUMMARY OF THE INVENTION nected to the electrohydraulic arrangement via a control cable comprised of a plurality of conductors, with means being provided for detecting the development of short circuits between such conductors and for responding by either shutting down the electrohydraulic arrangement or modifying its operation in such a manner as will not result in the development of a dangerous situation.

This object, and others which will become more understandable from the following description can be met, according to one advantageous concept of the invention by providing, in a remotely controlled electrohydraulic system, in combination, a hydraulic user device; a source of hydraulic fluid; a hydraulic arrange ment connecting said source of fluid to said user device and providing a path for the flow of fluid from said source to said user device, and including a control member positioned in said path and mounted for move: ment between a plurality of positions in different ones of which said control member differently affects the flow of fluid from said source to said user device, and

including a neutral position in which said control member blocks the flow of fluid from said source to said user device; electrohydraulic moving means operative when electricallyenergized for moving said control member by applying hydraulic force thereto; restoring means operative for applying to said control member a restoring force tending to move said member to said neutral position when said electrohydraulic moving means is electrically unenergized; electrical control means for controlling said electrohydraulic moving means, including a source of electrical power having two terminals, connecting means connecting said source of electrical power to said electrohydraulic moving means for furnishing electrical energy to the latter, a remote control arrangement and an electrical control cable having a first cable end connected to said electrohydraulic moving means and a second cable end connected to said remote control arrangement, said control cable comprising first conductor means having at said first cable end a first end connected to one terminal of said source of electrical power and having a second end at said second cable end, second conductor means extending along the length of said cable and having a first end at said first cable end and a second end at said second cable end; and fail-safe means for preventing electrical energization of said electrohydraulic moving means when a short-circuit develops between said first and second conductor means, to prevent the application by said electrohydraulic moving means of hydraulic force to said control member and to thereby cause said control member to be returned to said neutral position by said restoring means.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and. advantages thereof, will be best understood from the following description of specific embodiments when read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a first embodiment of the present invention; and

FIG. 2 illustrates a second embodiment of the invention. a

' DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a first embodiment of the system according to the invention. Reference numeral 1 designates in general an electrohydraulic control arrangement. The arrangement includes a four-port threeposition valve, generally designated with numeral 10. The valve 10 has a valve housing 12 provided with an inlet 13, an outlet flow conduit 14, and two output conduits l5, 16 for connection to a hydraulic user device. The valve housing 12 furthermore defines an internal bore 17 in which a three-land distributor piston 18 is seal-tightly mounted for sliding movement. Surrounding the three-land distributor piston 18, and axially spaced from each other, are an inflow chamber 19, two user-device outflow chambers 20, 21, and two returnflow outlet chambers 22, 23.

A pump 24 pumps pressure medium from a fluid reservoir 25 into the valve inlet 13. Two conduits 27, 28 connect respective ones of the two output conduits 15, 16 to respective inlets of a double-acting cylinder-andpiston arrangement 29, 30. The valve 10 is provided with means 31 for manually adjusting the axial position of the three-land distributor piston 18.

The valve housing 12 0f valve 10 has a right side wall 32 against which is positioned the left-side wall of the housing 33 of a pilot valve arrangement 11. The housing 33 is provided with an internal cylindrical chamber 34 in which is mounted for sliding movement a differential piston 35. The piston 35 divides the chamber 34 into a first chamber portion 36 located adjacent that axial end face of piston 35 having the smaller effective cross-sectional area, and into a second chamber portion 37 adjacent the axial end face of piston 35 having the larger effective cross-sectional area. A coupling 38 rigidly connects the differential piston 35 with the three-land distributor piston 18, so that the two can move together as a unit. Compression springs 39 and 40 are provided inthe'. chamber portions 36 and 37 and serve to bias the differential piston 35 and thereby piston 18 to their illustrated neutral positions. In the illustrated neutral position of distributor piston 18, the piston lands block off communication between the inflow chamber 19 and the two user-device outflow chambers 20, 21, and block off communication between the inflow chamber 19 and the two retum-flow outlet chambers 22 and 23.

Left-hand chamber 36 communicates via conduits 42, 43 and 50 with the fluid supply inlet 13 of valve 10. Also, left-hand chamber 36 communicates via conduit 42, two-port two-position valves 44 and 47, conduit 48, and conduit 49 with retum-flow outlet 14 of valve 10, which in turn communicates with the fluid reservoir 25 via an outlet conduit 26.

The right-hand chamber 37, on the other hand, communicates with fluid inlet 13 of valve 10, via conduit 46, two-port two-position valve 44, conduit 43, and conduit 50. Also, right-hand chamber 37 communicates with the fluid reservoir 25 via two-port twoposition valve 47, conduit 48, conduit 39, outflow conduit l4, and conduit 26.

Two-port two-position valves 44 and 47 are electromagnetically actuated two-position valves provided with biasing springs normally biasing them to their respective illustrated positions, in which they permit flow of fluid in the illustrated direction. When their respective electromagnets are energized, they are moved to their non-illustrated positions, in which they completely block the passage of fluid. The electromagnetic valves 44 and 47 are controlled over electrical control conductors 55, via a control switch 56, a voltage supply line 57 and a power supply line 58.

The voltage supply line 57 and the two control lines 55 run along the interior of a control cable 2, which connects the electrohydraulic control arrangement 55 with a remote control arrangement 3. The remote control arrangement includes a two-pole switch 56 comprised of two ganged switching members so constructed that when one of them is in the make position the other is in the break position, and vice versa. The powersupply line 58, connected directly to the non-illustrated power source, is connectable to and disconnectable from the voltage supply line 57 by means of two threshold switches 59, 60 here having the form of electromagnetic relays. Diodes 61 and 62 are respectively connected in parallel with the solenoids of relays 59 and 60.

One end of the winding of the first relay 59 is connected to the power supply line 58, while the other end of this winding is connected to the collector of an npn transistor 63 having an emitter connected to ground. The base of transistor 63 is connected to ground via a resistor 67, and is connected to a fail-safe switch 69 via a resistor 64, a Zener diode 65, and a fail-safe line 66.

The transistor 63 and the resistors 64, 67 together form a semiconductor switch, generally designated by numeral 68. The fail-safe line 66 travels along the length of the control cable 2 to the remote-control arrangement 3 and is there connected to the power supply line 58 via a fail-safe switch 69 and a resistor 70 connected in parallel with switch 69. It will be noted that the power supply cable 58 also travels from the power supply to the remote control arrangement 3, via the length of the control cable 2. The fail-safe conductor 66 may advantageously be an uninsulated, i.e., naked, conductor.

The magnetic winding of the second relay 60 is connected between groundand the fail-safe line 66.

The operation of the exemplary embodiment depicted in FIG. 1 is'as follows:

In its illustrated neutral position, the distributor piston 18 blocks the flow of fluid from valve inflow chamber 19 into the two inlets of the cylinder-and-piston arrangement 29, 30. The first valve 44 and the second valve 47 establish communication between their respective cylinder chamber portions 36, 37 on the one hand and the outflow conduit 14, on the other hand. Accordingly, there is no substantial build-up of fluid pressure on the two opposite faces of differential piston 35, and the biasing springs 39 and 40 can maintain the differential piston 35 in its illustrated neutral position.

When double-throw switch 56 is in its illustrated position, the solenoid of the second two-position valve 47 becomes energized, and this valve blocks fluid flow; the first two-position valve 44, however, remains open. Pressure fluid from inflow conduit 13 of valve 10 enter cylinder chamber 36, via conduits 50, 43 and 42, and also enters cylinder chamber 37, via conduits 50 and 43, via valve 44, and via conduits 45 and 46. With ap proximately equal pressures developed in the two cylinder chambers 36, 37, the differential piston 35 moves quickly to its leftmost position, i.e., towards valve 10.

If double-throw switch 56 is moved to its other (nonillustrated) position, then the first valve 44 blocks,

while the second valve 47 becomes unblocked. As a result, the differential piston 35 will move to its rightmost position, i.e., away from valve 10, because pressure fluid will be entering chamber 36 from fluid inlet 13, while pressure fluid can only leave chamber 37 via conduit 46, valve 47, and conduits 48, 49, 14 and 26, to be returned to the fluid reservoir 25.

When fail-safe switch 69 is closed, the full power supply potential is applied across the magnetic winding of the second relay 60. With such a voltage applied, the switch of relay 60 moves to its closed or conducting position, thereby connecting power supply line 58 with voltage supply line 57. lnasmuch as the voltage thereby applied to the cathode of Zener diode 65 will be higher than the breakdown voltage of this diode, Zener diode 65 will become conductive, with current flowing through the Zener diode, from cathode to anode, and to ground, via voltage-divider biasing resistor 64, 67, and also to ground via the base-emitter junction of transistor 63. Transistor 63 accordingly becomes conductive, energizing the magnetic winding of relay 59. Relay 59 is a break relay, while relay 60 is a make? relay. Accordingly, when relay 59 becomes energized, as it does now, its switch opens, thereby disconnecting the power supply line 58 from the voltage supply line 57.

When fail-safe switch 69 is open, a voltage is still applied across the winding of relay 60 from power supply line 58 via resistor 70. However, because of the voltage drop across the resistor 70, the voltage applied across relay 60 will notbe as high as it was when the relay was energized by closing of fail-safe switch 69. The threshold voltage of relay 60 is chosen below this lower voltage, and accordingly relay 60 becomes energized, even when fail-safe switch 69 is open. However, the lower voltage applied to the cathode of Zener diode 65 when fail-safe switch 69 is openis below the breakdown voltage of Zener diode 65, and accordingly Zener diode 65 does not conduct. As a result, transistor 63 does not conduct, and relay 59 is de-energized, so that its associated switch assumes the illustrated closed position. The switch of energized relay 60 is closed, and the switch of unenergized relay 59 is closed, and accordingly the power supply line 58 is electrically connected to the voltage supply line 57.

If a short-circuit develops between the fail-safe line 66 and one or both of the valve-control lines 55, the result will be the same as if the fail-safe switch 69 were closed. That is, the first relay 59 will become energized, and its respective switch will open, thereby terminating the electrical connection between power supply line 58 and voltage supply line 57. If one of the valves 44, 47 is blocked, and therefore energized, then if the respective energizing line55 becomes short-circuited to the fail-safe line 66, the energized valve becomes unblocked, and the piston 35 moves to its illustrated neutral position. On the other hand, if one of the valves, for example 47, is energized, and the fail-safe line 66 becomes short-circuited to the control line 55 for the other of the valves, i.e., becomes short-circuited to the control line 55 for valve 44, then the pistonwill remain in its selected position, and will return to its neutral position, as part of the fail-safe operation, only when switch 56 is subsequently moved in an attempt to reverse the energization of the valves 44, 47. In either event, however, there will be a return of the piston 35 to its neutral position, entirely blocking further flow of fluid into the inlets of user'device 29, 30.

On the other hand, if cable 2 becomes damaged in such a manner as to interrupt the continuity of either fail-safe line 66 or power supply line 58, this results in termination of current supply to the second relay 60, and accordingly the supply of energizing current via lines 55 to valves 44, 47 ends, and differential piston 35 returns to its illustrated neutral position.

Should the control cable 2 become damaged in some other manner, it is possible to nevertheless effect a return of piston 35 to its illustrated neutral position by closing fail-safe switch 69.

FIG. 2 depicts a second exemplary embodiment of the invention. The structure shown in FIG. 2 again includes a four-port three-position valve and a pilot valve arrangement 11, as shown in FIG. 1. However, in FIG. 2, the differential piston 35 of the pilot valve arrangement is provided with a position transducer 100, 101 which generates an electrical signal indicative of the axial displacement of the piston 35. The two electromagnetically actuated valves 44, 47, in this embodiment, are'connected to the output of power amplifiers 102, the inputs of which are connected to the two outputs of an electronic control stage 103. The electronic control circuitry for the control of valves 44, 47 furthermoreincludes, in this embodiment, a stage 1 04 which generates an analog signal indicative of the se lected axial position for the piston 35. As will be evident to persons skilled in the art, stages 101, 104 and 103 together form a conventional servo positioning system. Transducer 101 may include, for example, a potentiometer connected across a voltage source and having a wiper connected to the linkage rod 100, so that movement of the linkage rod will vary the voltage appearing at the potentiometer tap in a corresponding manner. Thus, stage 101 generates for example a volt age indicative of the actual position of the piston 35, whereas stage 104 generates a voltage indicative of the desired position for the piston 35. Stage 103 can be a difference amplifier for forming the difference of the two voltages supplied by stage 101 and 104. Such difference amplifier can have its output connected to a first half-wave rectifier having an output connected to one of the two amplifiers 102, and the difference amplifier can have its output further connected to a second half-wave rectifier, of different polarity than the first, having its respective output connected to the other of the two amplifiers 102. Accordingly, if the difference in the voltages at the outputs of stages 101 and 104 is of a first polarity and of sufficient magnitude, then one of valves 44, 47 will become energized. On the other hand, if the just-mentioned voltage difference is of opposite second polarity and of sufficient magnitude, then the-other of valves 44, 47 will become energized.

The fail-safe arrangement in the embodiment of FIG. 2 corresponds in many respects to the fail-safe arrangement in the embodiment of FIG. 1.

In FIG. 2, however, the collector of transistor 63 is connected to the power supply line 58 by way of a resistor 105, instead of via the relay 59 in FIG. 1. Also, the collector of transistor 63 in FIG. 2 is connected, via two diodes 106, with the outputs of valve-control amplifiers 102. Also in this embodiment, there are two voltagesupply lines 57, which are connected not directly to the electrical inputs of valves 44, 47, but rather are connected to the valve-control amplifiers 102 and furnish the biasing voltages for such amplifiers 102.

Also, in the embodiment of FIG. 2, the remote control arrangement 3 is provided with a potentiometer 107. By varying the position of the potentiometer wiper, it is possible to select the desired axial position for the differential piston 35. Specifically, the two ends and the wiper of potentiometer 107 are connected, via three lines 55, to three corresponding inputs of stage 104, which may for example contain a voltage source across which the end terminals of potentiometer 107 may be connected, with the output of stage 104 being directly connected, for example, to the center input of stage 104.

The operation of the arrangement shown in FIG. 2 is evident from the foregoing description. The wiper of potentiometer 107 is adjusted, to select the desired position for piston 35. The control arrangement 103 compares the actual and desired values of the position of piston 105, and energizes one of valves 44, 47, to cause the piston 35 to move to the position selected therefor, in conventional servo fashion.

The operation of the fail-safe circuitry in the embodi- 102 are shorted to ground via diodes 106 and transistor 63. Accordingly, substantially no energizing current will flow into the electromagnetic windings of valves 44 and 47. As a result, valves 44 and 47 will both bein the open position (permitting the flow of fluid), and the differential piston 35, as was the case in the embodiment of FIG. 1, will assume its illustrated neutral position. In FIG. 2 resistor 105 is provided for currentlimiting purposes. In the event of an interruption in the continuity of power supply line 58 in the control cable 2, the biasing voltage normally applied to valve-control amplifiers 102 via biasing lines 57 will be removed, thereby causing both valves 44, 47 to be unenergized, and thus causing a return of piston 35 to its illustrated neutral position. ln the event of an interruption in the fail-safe line 66 in the cable 2, relay 60 will become deenergized, and its associated switch will open, likewise removing the biasing voltage normally applied to amplifiers 102, and again the piston 35 will return to its illustrated neutral position.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of circuits and constructions differing from the types described above.

While the invention has been illustrated and described as embodied in a remotely controlled electrohydraulic arrangement provided with fail-safe features, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. I

Without further analysis, the foregoing will so fully reveal the gistof the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In a remotelycontrolled electrohydraulic system, in combination, a hydraulic user device; a source of hydraulic fluid; a hydraulic arrangement connecting said source offluid to said user device and providing a path for the flow of fluid from said source to said user device, and including a control member positioned in said path and mounted for movement between a plurality of positions in different ones of which said control member differently affects the flow of fluid from said source to said user device, and including a neutral position in which said control member blocks the flow of fluid from said source to said user device; electrohydraulic moving means operative when electrically energized for moving said control member by applying hydraulic force thereto; restoring means operative for applying to said control member a restoring force tending to move said member to said neutral position when said electrohydraulic moving means is electrically unenergized; electrical control means for controlling said electrohydraulic moving means, including a source of electrical power having two terminals, connecting means connecting said source of electrical power to said electrohydraulic moving means for furnishing electrical energy to the latter, a remote control arrangement and an electrical control cable having a first cable end connected to said electrohydraulic moving means and a second cable end connected to said remote control arrangement, said control cable comprising first conductor means having at said first cable end a first end connected to one terminal of said source of electrical power and having a second end at said second cable end, second conductor means extending along the length of said cable and having a first end a said first cable end and a second end at said second cable end; and fail-safe means for preventing electrical energization of said electrohydraulic moving means when a short-circuit develops between said first and second conductor means, to prevent the application by said electrohydraulic moving means of hydraulic force to said control member and to thereby cause said control.

member to be returned to said neutral position by said restoring means.

2. A system as defined in claim 1, wherein said failsafe means comprises voltage-responsive means connected to said first end of said second conductor means and operative for detecting the development of a shortcircuit between said first and second conductor means by detectingthe resulting change in voltage atsaid first end of said second conductor means.

3. A system as defined in claim 2, wherein said voltage-responsive means comprises a Zener diode circuit including a Zener diode connected to said first end of said second conductor means in such a manner as to break down and become conductive in responsive to a rise of the voltage at said first end of said second conductor means resulting from the development of a short-circuit between said first and second conductor means, and disconnecting means connected to said Zener diode in such a manner as to interrupt the connection between said source of electrical energy and said electrohydraulic means in response to break down of said Zener diode.

4. A system as defined in claim 3, wherein said failsafe means comprises impedance means connecting together said second ends of said first and second conductor means, so that a predetermined lower voltage will exist at said first end of said second conductor means when said first and second conductor means are not short-circuited to each other, and so that a predetermined higher voltage will exist at said first end of said second conductor means when said first and second conductor means are short-circuited to each other, and wherein the voltage applied to the cathode of said Zener diode when said first and second conductor means are not short-circuited to each other is such as to render said Zener diode conductive, and wherein the voltage applied to the cathode of said Zener diode when said first and second conductor means are shortcircuited to each other is insufficient to render said Zener diode conductive.

5. A system as defined in claim 4, wherein said failsafe means further includes a fail-safe switch operative for short-circuiting said first and second conductor means to each other, to cause said control member to assume said neutral position.

6. A system as defined in claim 5, wherein said electrohydraulic moving means comprises a differential cylinder-and-piston unit having inlets and outlets for the flow of hydraulic fluid and a plurality of electrically energized valves for controlling the flow of fluid through such inlets and outlets, and wherein said connecting means comprises a plurality of valve-control conductors in said cable, one valve-control conductor for each of said electrically energized valves, with each valve-control conductor extending between a respective one of said electrically energized valves and said remote control arrangement, and wherein said control means includes an additional voltage supply conductor having a first end connected to said one terminal of said source of electrical energy and having a second end at said second cable end, and wherein said remote control arrangement comprises switch means for connecting said voltage supply conductor to a selected one of said valve-control conductors, to energize a selected one of said electrically energized valves, with any shortcircuiting between said second conductor means and said voltage supply conductor resulting in breakdown of said Zener diode, and with any short-circuiting between said second conductor means and any energized one of said valve-control conductors also resulting in breakdown of said Zener diode.

7. A system as defined in claim 1, wherein said failsafe means comprises means for preventing electrical energization of said electrohydraulic moving means when a short-circuit develops between said first and second conductor means by shunting energizing current away from said electrohydraulic moving means.

8. A system as defined in claim 1, wherein said failsafe means further includes means for preventing electrical energization of said electrohydraulic moving means in response to an interruption in the continuity of one of said first and second conductor means.

9. A system as defined in claim 1, wherein said electrohydraulic moving means comprises electrohydraulic servo positioning means for varying the position of said control member, said servo positioning means comprising electronic circuitry requiring a biasing voltage for operation, and wherein said connecting means comprises means connected to said source of electrical energy for applying a biasing voltage to such electronic circuitry, and wherein said fail-safe means comprises,

means for tenninating the application of such biasing voltage to said electronic circuitry in response to an interruption in the continuity of one of said first and second conductor means. 

1. In a remotely controlled electrohydraulic system, in combination, a hydraulic user device; a source of hydraulic fluid; a hydraulic arrangement connecting said source of fluid to said user device and providing a path for the flow of fluid from said source to said user device, and including a control member positioned in said path and mounted for movement between a plurality of positions in different ones of which said control member differently affects the flow of fluid from said source to said user device, and including a neutral position in which said control member blocks the flow of fluid from said source to said user device; electrohydraulic moving means operative when electrically energized for moving said control member by applying hydraulic force thereto; restoring means operative for applying to said control member a restoring force tending to move said member to said neutral position when said electrohydraulic moving means is electrically unenergized; electrical control means for controlling said electrohydraulic moving means, including a source of electrical power having two terminals, connecting means connecting said source of electrical power to said electrohydraulic moving means for furnishing electrical energy to the latter, a remote control arrangement and an electrical control cable having a first cable end connected to said electrohydraulic moving means and a second cable end connected to said remote control arrangement, said control cable comprising first conductor means having at said first cable end a first end connected to one terminal of said source of electrical power and having a second end at said second cable end, second conductor means extending along the length of said cable and having a first end a said first cable end and a second end at said second cable end; and fail-safe means for preventing electrical energization of said electrohydraulic moving means when a short-circuit develops between said first and second conductor means, to prevent the application by said electrohydraulic moving means of hydraulic force to said control member and to thereby cause said control member to be returned to said neutral position by said restoring means.
 2. A system as defined in claim 1, wherein said fail-safe means comprises voltage-responsive means connected to said first end of said second conductor means and operative for detecting the development of a short-circuit between said first and second conductoR means by detecting the resulting change in voltage at said first end of said second conductor means.
 3. A system as defined in claim 2, wherein said voltage-responsive means comprises a Zener diode circuit including a Zener diode connected to said first end of said second conductor means in such a manner as to break down and become conductive in responsive to a rise of the voltage at said first end of said second conductor means resulting from the development of a short-circuit between said first and second conductor means, and disconnecting means connected to said Zener diode in such a manner as to interrupt the connection between said source of electrical energy and said electrohydraulic means in response to break down of said Zener diode.
 4. A system as defined in claim 3, wherein said fail-safe means comprises impedance means connecting together said second ends of said first and second conductor means, so that a predetermined lower voltage will exist at said first end of said second conductor means when said first and second conductor means are not short-circuited to each other, and so that a predetermined higher voltage will exist at said first end of said second conductor means when said first and second conductor means are short-circuited to each other, and wherein the voltage applied to the cathode of said Zener diode when said first and second conductor means are not short-circuited to each other is such as to render said Zener diode conductive, and wherein the voltage applied to the cathode of said Zener diode when said first and second conductor means are short-circuited to each other is insufficient to render said Zener diode conductive.
 5. A system as defined in claim 4, wherein said fail-safe means further includes a fail-safe switch operative for short-circuiting said first and second conductor means to each other, to cause said control member to assume said neutral position.
 6. A system as defined in claim 5, wherein said electrohydraulic moving means comprises a differential cylinder-and-piston unit having inlets and outlets for the flow of hydraulic fluid and a plurality of electrically energized valves for controlling the flow of fluid through such inlets and outlets, and wherein said connecting means comprises a plurality of valve-control conductors in said cable, one valve-control conductor for each of said electrically energized valves, with each valve-control conductor extending between a respective one of said electrically energized valves and said remote control arrangement, and wherein said control means includes an additional voltage supply conductor having a first end connected to said one terminal of said source of electrical energy and having a second end at said second cable end, and wherein said remote control arrangement comprises switch means for connecting said voltage supply conductor to a selected one of said valve-control conductors, to energize a selected one of said electrically energized valves, with any short-circuiting between said second conductor means and said voltage supply conductor resulting in breakdown of said Zener diode, and with any short-circuiting between said second conductor means and any energized one of said valve-control conductors also resulting in breakdown of said Zener diode.
 7. A system as defined in claim 1, wherein said fail-safe means comprises means for preventing electrical energization of said electrohydraulic moving means when a short-circuit develops between said first and second conductor means by shunting energizing current away from said electrohydraulic moving means.
 8. A system as defined in claim 1, wherein said fail-safe means further includes means for preventing electrical energization of said electrohydraulic moving means in response to an interruption in the continuity of one of said first and second conductor means.
 9. A system as defined in claim 1, wherein said electrohydraulic moving means comprises electrohydraulic servo positionIng means for varying the position of said control member, said servo positioning means comprising electronic circuitry requiring a biasing voltage for operation, and wherein said connecting means comprises means connected to said source of electrical energy for applying a biasing voltage to such electronic circuitry, and wherein said fail-safe means comprises means for terminating the application of such biasing voltage to said electronic circuitry in response to an interruption in the continuity of one of said first and second conductor means. 